Torque fluctuation, that is, slot ripple, occurs during the rotation of a rotor due to the presence of slots in a stator in a synchronous motor provided with a stator having slots for a winding, and a rotor having permanent magnets. This is due to a change in clearance between the rotor and the stator in correspondence with an angular position of the rotor, because of the slots in the stator, and a consequent change of density of magnetic flux. A slight fluctuation of feed velocity occurs when an electric motor having slot ripple is used, for example, in a feed mechanism of a machine tool, and thus the finished accuracy of a workpiece is poor.
To solve the above problem, the applicant disclosed, in Japanese Unexaminated Patent Publication No. 63-178750, a rotor structure wherein the rotor is divided into a plurality of unitary rotor elements arranged along an axis thereof, and each unitary rotor element is fixed at a first angular position or a second angular position separated from the first angular position by a prescribed angle.
Seventy two slot ripples occur in one rotation of a rotor of, for example, an eight pole-synchronous motor having thirty six slots, and in this case, the angular difference between the above-mentioned first and second angular positions is 2.5 degrees, but an angular error of as much as one degree can occur during the manufacture and assembly of a rotor. In the above-mentioned rotor having two angular positions, when the error angle is 2.5 degrees, each slot ripple caused by a relative position between the stator slots and each unitary rotor element fixed at the first or second angular position has the same phase, and thus a slot ripple similar to a ripple in the prior art rotor not having a skew structure in which each angular position is shifted, occurs. Although the error in the angle between the two angular positions in a skew rotor is not necessarily exactly 2.5 degrees, the effect obtained by adopting the skew structure is substantially lost when an angular error of as much as one degree occurs